This invention relates to fibrous assemblies, and to a process for their preparation. More specifically, this invention relates to paper products, which find utility as toweling and sanitary products, and to a process for their preparation. Said products are characterized by enhanced wet strength, absorbency, softness, good drape, and enhanced bulk which exhibits compaction resistance, that is, a bulk which persists substantially undiminished even though the product be wetted.
Further, this invention relates to a process in which the above-described products can be obtained from relatively short fibers. For example, with respect to papermaking cellulosic fibers, the term short fibers is descriptive of fibers originating from the so-called hardwoods, i.e., the angiosperms, where the fibers typically have a length ranging from 1.0 to 2.0 mm. Such woods are normally pulped by the sulphite process. This is to be contrasted with long fibers which originate, for example, from the so-called soft woods, i.e., the gymnosperms, which typically have fiber lengths ranging from about 3..5 to 5.0 mm; such woods are normally pulped by the more expensive Kraft process.
While cellulosic fibers have been specifically named it is to be emphasized at the outset that the present invention contemplates all fibers: natural, synthetic, or blends thereof. Also it is to be emphasized that while economic considerations make utilization of short fibers most attractive, this invention is not restricted to short fibers but fully contemplates all fibers typically employed in the paper, felt, and allied arts; that is, plant fibers, such as cotton, esparto, straw, wood, etc.; synthetic fibers, such as rayon, nylon, glass, etc.; animal fibers, such as wool, fur, hair, silk, etc.; and mineral fibers, such as asbestos.
With respect to the paper product embodiment of this invention the prior art has essentially proceeded along two approaches in the obtainment of paper products which are characterized as having wet-strength, bulkiness, and water absorbency. These two approaches are: (1) machine process means, and (2) cmpositional variables, such as the particular classes of fiber pulp employed and the identity of chemical additives, where said additives are predominantly a means to enhance wet-strength.
With respect to the addition of chemical agents to enhance wet-strength, the following patents are representative of the art: U.S. Pat. No. 3,058,873, granted Oct. 16, 1962, and Canadian Pat. No. 828,656, granted Dec. 2, 1969. These patents generally teach the sequential or simultaneous addition of various chemical agents to the pulp furnish prior to web formation. These agents can be divided into two basic categories: anionic and cationic. Typical of cationic agents are metal salts, such as alum, and organic compounds, such as urea-formaldehyde and melamine-formaldehyde resins, polyalkylene polyamines, polyamides and derivatives thereof such as polyamide-epichlorohydrin reaction products. Typical of the anionic agents are clays, such as bentonite and various gums, starches, vinyl copolymers of carboxylic acids, and cellulose derivatives, such as carboxymethylcellulose.
Whatever the identity of the additive or combination of additives, it is generally recognized that the process technique of developing wet-strength by the addition of chemical additives is satisfactory in most part, but there are certain difficulties. For example, in the manufacture of wet-strength papers an appreciable to significant amount of the wet-strength agent, added at a point upstream from sheeting (e.g., at the beater, pulp chest, or headbox), is lost in the white water. Another difficulty results when anionic and cationic agents are added, since to a significant extent these agents tend to agglomerate or react with each other without fiber deposition and, therefore, do not impart any functional benefit to the ultimate paper product. Also, to date, no method of enhancing wet-strength by addition of chemical additives functions to enhance the softness or bulkiness of the resultant paper product. Indeed, use of wet-strength agents almost invariably causes increased harshness and reduced softness. Bulkiness, softness and water absorbency are related characteristics and to date these characteristics have been imparted to paper products largely by processing means which require special equipment.
U.S. Pat. No. 3,301,746, granted Jan. 31, 1967, illustrates one process means for preparing bulky paper sheets having a highly desirable combination of softness, bulk and absorbency characteristics such that the resulting product is ideally suited for toweling and sanitary purposes. These desirable characteristics are engendered by a particular technique used in the formation, transfer and drying of the advancing paper web such that mechanical compaction is limited to a repeating point array on the surface of the web.
Thus, conventional process means of enhancing bulkiness of a paper product are largely operational features which minimize compression of the continuous web prior to final drying; for it is well-known in the art that substantial compression of the web prior to final drying irreversibly reduces the caliper of the resulting paper product. Also, common to most prior art procedures for the production of bulky paper from wood pulp is a requirement that a substantial proportion of the fibers have a length of approximately 3 millimeters, that is, are classifiable in the art as long fibers as opposed to short fibers which typically have a fiber length of approximately 1 to 2 mm. This common requirement reflects the fact that in bulky paper products the forces holding the fibers intact are largely that of a physical nature where ultimate tensile strength is directly proportional to fiber length, i.e., the strength is derived from a mechanical entanglement of the long fibers which cannot be achieved with a furnish composed of substantially all short fibers.
Accordingly, it is an object of this invention to provide a process wherein short papermaking fibers acquire the desirable characteristics of long papermaking fibers.
A second object of this invention is to provide bulky fibrous assemblies, such as paper sheets, wherein short fibers are present as discrete fiber aggregates as at least a substantial component thereof.
A third object of this invention is to provide a process whereby bulky fibrous assemblies, such as paper sheets, can be manufactured from short fibers. It should be noted that while this invention is primarily concerned with the utilization of short fibers for economic reasons the invention contemplates the use of fibers of any length whether the fibers be naturally occurring or synthetic.
A fourth object of this invention is to provide fibrous assemblies, such as bulky paper products exhibiting a characteristic softness impression which also have enhanced wet-strength.
A fifth object of this invention is to provide a process for the production of fibrous assemblies, such as bulky paper products, which exhibit wet compaction resistance, that is, are possessed of a caliper which is substantially independent of the state of hydration of said fibrous assemblies; a complementary object is a product characterized in part by such wet compaction resistance.